Interpretive Summary: Streamflow generated by snowmelt is traditionally thought to originate by overland flow to a stream. Data collected at the Upper Sheep Creek Watershed located within the Reynolds Creek Experimental Watershed were analyzed to characterize the interactions between snowmelt, groundwater and streamflow. The authors illustrated that snowmelt recharge to shallow groundwater systems is the primary source of streamflow in a typical mountainous watershed, and that snowmelt can be delivered to the stream channel via subsurface flow much more rapidl than conventionally assumed. Time between snowmetl and response in groundwater and streamflow varied from year to year depending on whether snowmelt was sufficient to fill the subsurface groundwater system. Many hydrologic models currently used to not address the contribution of groundwater to streamflow and are unable to predict this observed phenonmenon. This research highlights the need for groundwater and subsurface flow routines in hydrologic models. A conceptual model of the groundwater flow system was developed for subsequent modeling efforts.

Technical Abstract:
Snowmelt recharge to shallow groundwater systems is the primary source of streamflow in many mountainous watersheds, but characteristics of these systems are not wel understood, and their contribution to streamflow is often not appreciated. Data from a detailed study on the Upper Sheep Creek Watershed located in the Reynolds Creek Experimental Watershed in southwestern Idaho, were analyzed to characterize the interactions between snowmelt, groundwater and streamflow in this mountainous watershed. Response time between the snowmelt, groundwater levels and steamflow was drastically different from year to year depending on the extent of the snowpack. Response time to snowmelt for piezometers located 135 m downslope of an isolated drift was two to three days during years of average snow accumulation and up to 42 days for a year with snow accumulation 40% of normal. A conceptual model of the basin was formulated dfor subsequent modling efforts. The primary aquifer is believed to be unconfined during low snowmelt years and confined when groundwater levels are high because of normal or above normal snowmelt. Snowmelt from an isolated drift enters the primary aquifer upslope from where the confining layer is discontinuous. Rapid response during years with normal snow accumulation is therefore primarily a pressure pulse throught the primary aquifer. Recharge druing years of low snow accumulation is insufficient to fill the primary aquifer to the confining layer, and response time is indicative of travel time through the aquifer.